Semiconductor memory cell having information storage transistor and switching transistor

ABSTRACT

A semiconductor memory cell, or a semiconductor memory cell for ASICs, of the structure is provided which ensures stable transistor operation, which does not require a large-capacitance capacitor as required in conventional DRAMs, which ensures reliable reading and writing of information, that permits short-channel design, and that allows the cell area to be reduced. The semiconductor memory cell includes: an information storage transistor TR 1  comprising a semiconductor channel layer Ch 1 , first and second conductive gates G 1 , G 2 , and first and second conductive layers L 1 , L 2  ; and a switching transistor TR 2  comprising a semiconductor channel forming region Ch 2 , a third conductive gate G 3 , and third and fourth conductive layers L 3 , L 4 , wherein the fourth conductive layer L 4  is connected to the second conductive gate G 2 , the first conductive gate G 1  and the third conductive gate G 3  are connected to a first memory-cell-selection line, the first conductive layer L l  and the third conductive layer L 3  are connected to a second memory-cell-selection line, the second conductive layer L.sub. 2 is connected to a fixed potential, and the semiconductor channel forming region Ch 2  is connected to a read/write selection line.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor memory cell consisting of two transistors or one transistor formed by combining two transistors.

2. Description of the Prior Art

FIG. 17 shows a conventional dynamic memory cell consisting of a single transistor and a capacitor; the memory cell of this structure is generally known as a one-transistor memory cell and used as a semiconductor memory cell suitable for high density devices. In such a memory cell, the capacitance of the capacitor needs to be high enough to cause a voltage change on the associated bit line. However, the trend to smaller plan area of the semiconductor memory cell necessitates a reduction in the size of the capacitor formed in a horizontal plate-like shape, giving rise to the problem that when reading information stored as a charge on the memory cell capacitor, the information is masked by noise, or that, because of increasing stray capacitance on the bit line with each generation of memory cells, only a small voltage change can be caused to the bit line. In one approach to solving this problem, there is proposed a dynamic memory cell having a trench capacitor cell structure (see FIG. 18) or a stacked capacitor cell structure. However, because of processing limitations on how deep the trench can be formed and how high the stack (stacked layers) can be made, there is a limit to increasing the capacitance of the capacitor. Accordingly, it is said that dynamic memory cells of these structures encounter the limit of miniaturization in the dimensional region beyond the low submicron rule.

Furthermore, with transistors forming the semiconductor memory cells, reducing the transistor's plan area beyond the low submicron rule would introduce such problems as dielectric strength degradation, punch-through, etc., which would increase the possibility of leakage even under the rated voltage condition. With conventional transistor structures, therefore, it becomes difficult to ensure proper operation of the memory cell when the memory cell size is reduced.

To overcome the above limitations on the capacitance, there is proposed a memory cell structure wherein the memory cell is constructed with two transistors and the transistor channel current is sensed.

In the memory cell disclosed in Japanese Patent Unexamined Publication No. 63-19847, for example, a capacitor C₂ coupled to the gate and drain of a MOS transistor Q₁ is connected to a word line, as shown in FIG. 3 accompanying the same Patent Publication. Furthermore, the drain of the transistor Q₁ is connected to the gate of an SOI transistor Q₂ of the complementary type to the transistor Q₁. The drain of the transistor Q₂ is in turn connected to a fixed potential V_(D), while the source of each of the transistors, Q₁ and Q₂, is connected to a bit line.

In the memory cell disclosed in the above Patent Publication, since the gate electrode of the SOI (silicon-on-insulator) transistor Q₂ is formed only on one principal surface side of the channel region thereof having two principal surfaces, an extra capacitor C₂ is needed to complete the structure of the memory cell. A further problem is that because of the charge or electric field applied (through an insulating film) to the other principal surface of the channel region of the SOI transistor Q₂, the operation of the SOI transistor Q₂ becomes unstable and it is difficult to reduce the channel length.

On the other hand, the memory cell disclosed in Japanese Patent Unexamined Publication No. 1-145850 is constructed with a write transistor 18, a read transistor 19 (an SOI transistor), and a protective capacitor 20, as shown in FIG. 1 accompanying the same Patent Publication. The source of the write transistor 18 is connected to the gate of the read transistor 19. The cathode of the protective diode 20 is connected to the drain of the read transistor 19. Further, the anode of the protective diode 20 and the drain of the write transistor 18 are connected to a bit line, while the source of the read transistor 19 and the gate of the write transistor 18 are connected to a word line.

In the memory cell disclosed in the above Patent Publication, since the read transistor 19 is an SOI transistor, when the source potential is caused to change to the same polarity as the gate potential (when writing a "1") the potential of the channel region changes with the change of the gate potential, resulting in an incomplete writing condition. It is therefore difficult to read the "1" with the expected certainty. Furthermore, in the read transistor 19, the gate electrode is formed only on one principal surface side of the channel region having two principal surfaces. Therefore, this structure also has the problem that because of the charge or electric field applied (through an insulating film) to the other principal surface side of the channel region of the read transistor 19, the operation of the read transistor 19 becomes unstable and it is difficult to reduce the channel length.

The memory cells disclosed in Japanese Patent Unexamined Publication Nos. 62-141693 and 62-254462 each comprise a write transistor T1, a read transistor T2, and a storage capacitor C, as shown in FIG. 1 accompanying the former Patent Publication. The drains of the transistors T1 and T2 are connected to a bit line, and the source of the transistor T1 is connected to the storage capacitor and also to a first gate of the transistor T2. Further, the gate of the transistor T1 is connected to a write selection line, while a second gate (or a channel forming region such as a well) of the transistor T2 is connected to a read selection line.

When the read transistor T2 is formed from an SOI transistor, the first and second gates are respectively formed on the upper and lower surface sides of the channel region of the transistor T2 (refer to Japanese Patent Application No. 55-93521 and Japanese Patent Unexamined Publication No. 57-18364). This structure eliminates the problem that has placed a limitation on the reduction of the channel length. For the write transistor T1, on the other hand, only bulk-type transistors are disclosed as preferred embodiments, and therefore, there is a limit to the miniaturization of the memory cell as a whole. Furthermore, the structure requiring each word line to be divided into a read line and a write line has the problem of increased chip area or increased number of stacked layers.

The present invention is concerned with the structure of a memory cell constructed with two transistors and yet capable of solving the above enumerated problems. An object of the invention is to provide a semiconductor memory cell, a semiconductor memory cell for ASICs (Application Specific Integrated Circuits), and even a semiconductor memory cell with a single transistor formed by combining two transistors, of the structure that ensures stable transistor operation, that does not require the provision of a large-capacitance capacitor as required in prior art DRAMs, and that allows the channel length to be reduced and achieves miniaturization of the cell.

SUMMARY OF THE INVENTION

To achieve the above object, in accordance with a first aspect of the invention, there is provided a semiconductor memory cell, as shown in the schematic diagram of FIG. 1, comprising:

an information storage transistor TR₁ comprising a semiconductor channel layer Ch₁ having first and second opposing principal surfaces; first and second conductive gates, G₁ and G₂, respectively disposed opposite the two principal surfaces of the semiconductor channel layer Ch₁ with first and second barrier layer respectively interposed therebetween; and first and second conductive regions, L₁ and L₂, respectively connected to either end of the semiconductor channel layer Ch₁, and

a switching transistor TR₂ comprising a semiconductor channel forming region Ch₂ having a third principal surface; a third conductive gate G₃ disposed opposite the third principal surface of the semiconductor channel forming region Ch₂ with a third barrier layer interposed therebetween; and third and fourth conductive layers, L₂ and L₄, each formed in a surface region of the semiconductor channel forming region Ch₂ and near either end of the third conductive gate G₃, wherein

the fourth conductive layer L₄ is connected to the second conductive gate G₂,

the first conductive gate G₁ and the third conductive gate G₃ are connected to a first memory-cell-selection line,

the first conductive layer L₁ and the third conductive layer L₃ are connected to a second memory-cell-selection line,

the second conductive layer L₂ is connected to a fixed potential including zero potential, and

the semiconductor channel forming region Ch₂ is connected to a read/write selection line.

In the above structure, the conductive layers may be formed from a low-resistivity semiconductor, a silicide, a two-layered structure of silicide and semiconductor, a metal, or the like. The barrier layers serve as barriers to channel carriers, and may be formed from an insulating material or a wide-gap semiconductor material.

In one preferred mode of the semiconductor memory cell according to the first aspect of the invention, as shown in the schematic diagram of FIG. 4,

the information storage transistor TR₁ is formed from a transistor of a first conductivity type,

the switching transistor TR₂ is formed from a transistor of a conductivity type opposite to the first conductivity type,

the first conductive layer L₁ is connected to the second line via a fifth conductive layer L₅ which forms a rectifier junction with the first conductive layer L₁, and

the semiconductor channel forming region Ch₂ is connected to a second fixed potential including zero potential.

To achieve the above object, in accordance with a second aspect of the invention, there is provided a semiconductor memory cell, as shown in the schematic diagram of FIG. 7, comprising:

an information storage transistor TR₁ comprising a first semiconductor channel layer Ch₁ having first and second opposing principal surfaces; first and second conductive gates, G₁ and G₂, respectively disposed opposite the two principal surfaces of the first semiconductor channel layer Ch₁ with first and second barrier layers respectively interposed therebetween; and first and second conductive regions, L₁ and L₂, each connected to either end of the first semiconductor channel layer Ch₁, and

a switching transistor TR₂ comprising a second semiconductor channel layer Ch₂ having third and fourth opposing principal surfaces; third and fourth conductive gates, G₃ and G₄, respectively-disposed opposite the two principal surfaces of the second semiconductor channel layer Ch₂ with third and fourth barrier layers respectively interposed therebetween; and third and fourth conductive layers, L₃ and L₄, each connected to either end of the second semiconductor channel region Ch₂, wherein

the fourth conductive layer L₄ is connected to the second conductive gate G₂,

the first conductive gate G₁ and the third conductive gate G₃ are connected to a first memory-cell-selection line,

the first conductive layer L₁ and the third conductive layer L₃ are connected to a second memory-cell-selection line,

the second conductive layer L₂ is connected to a fixed potential including zero potential, and

the fourth conductive gate G₄ is connected to a read/write selection line.

In one preferred mode of the semiconductor memory cell according to the second aspect of the invention, as shown in the schematic diagram of FIG. 10,

the information storage transistor TR₁ is formed from a transistor of a first conductivity type,

the switching transistor TR₂ is formed from a transistor of a conductivity type opposite to the first conductivity type,

the first conductive layer L₁ is connected to the second line via a fifth conductive layer L₅ which forms a rectifier junction with the first conductive layer L₁, and

the fourth conductive gate G₄ is connected to a second fixed potential including zero potential.

To achieve the above object, in accordance with a third aspect of the invention, there is provided a semiconductor memory cell, as shown in the schematic diagram of FIG. 12(A), comprising:

an information storage transistor TR₁ of a first conductivity type, comprising a first semiconductor channel forming region Ch₁ ; a first conductive gate G₁ formed above the surface thereof with a first barrier layer interposed therebetween; a first conductive region SC₁ ; and a second conductive region SC₂, and

a switching transistor TR₂ of a second conductivity type opposite to the first conductivity type, comprising a second semiconductor channel forming region Ch₂ ; a second conductive gate G₂ formed above the surface thereof with a second barrier layer interposed therebetween; a third conductive region SC₃ ; and a fourth conductive region SC₄, wherein

the first conductive gate G₁ of the information storage transistor TR₁ and the second conductive gate G₂ of the switching transistor TR₂ are connected to a first memory-cell-selection line;

the fourth conductive region SC₄ of the switching transistor TR₂ is connected to the first semiconductor channel forming region Ch₁ of the information storage transistor TR₁ ;

the third conductive region SC₃ of the switching transistor TR₂ is connected to a second memory-cell-selection line, and

the first conductive region SC₁ of the information storage transistor TR₁ is connected to a read line.

The second conductive region SC₂ of the information storage transistor TR₁ is supplied with a fixed potential including zero potential. Preferably, the first conductive region SC₁ is connected to the second line or the third conductive region SC₃ via a rectifier junction. Preferably, the first conductive region SC₁ is formed from a semiconductor as a common region with the second semiconductor channel forming region Ch₂, the rectifier junction being formed between the common region and the third conductive region SC₃, and the first semiconductor channel forming region Ch₁ and the fourth conductive region SC₄ are formed from a common region. Furthermore, the second conductive gate G₂ may be formed common with the first conductive gate G₁, as in a fifth aspect of the invention hereinafter described. In this case, it is desirable that the third conductive region SC₃ further include a metal layer of Mo, Al, or the like, or a silicide layer that forms a Schottky junction with the first conductive region SC₁.

In the above structure, the conductive regions may be formed from a low-resistivity semiconductor, a silicide, a two-layered structure of silicide and semiconductor, a metal, or the like.

To achieve the above object, in accordance with a fourth aspect of the invention, there is provided a semiconductor memory cell, as shown in the schematic diagram of FIG. 14(A), comprising:

an information storage transistor TR₁ of a first conductivity type, comprising a first semiconductor channel forming region Ch₁ ; a first conductive gate G₁ formed above the surface thereof with a first barrier layer interposed therebetween; a first conductive region SC₁ ; and a second conductive region SC₂, and

a switching transistor TR₂ of a second conductivity type opposite to the first conductivity type, comprising a second semiconductor channel forming region Ch₂ ; a second conductive gate G₂ formed above the surface thereof with a second barrier layer interposed therebetween; a third conductive region SC₃ ; and a fourth conductive region SC₄, wherein

the first conductive gate G₁ of the information storage transistor TR₁ and the second conductive gate G₂ of the switching transistor TR₂ are connected to a first memory-cell-selection line;

the fourth conductive region SC₄ of the switching transistor TR₂ is connected to the first semiconductor channel forming region Ch₁ of the information storage transistor TR₁ ;

the third conductive region SC₃ of the switching transistor TR₂ is connected to a second memory-cell-selection line,

the second conductive region SC₂ of the information storage transistor TR₁ is connected to a fixed potential, and

the first conductive region SC₁ of the information storage transistor TR₁ is connected to the third conductive region SC₃ of the switching transistor TR₂, forming a rectifier junction therebetween.

A third conductive gate G₃ may be further provided opposite a third principal surface of the second semiconductor channel forming region Ch₂ with a third barrier layer interposed therebetween.

To achieve the above object, in accordance with a fifth aspect of the invention, there is provided a semiconductor memory cell, as shown in the schematic diagram of FIG. 15(A), comprising:

a first semiconductor region SC₁ of a first conductivity type formed in a surface region of a semiconductor substrate or on an insulating substrate,

a first conductive region SC₂ formed in a surface region of the first semiconductor region SC₁ in contacting relationship forming a rectifier junction therebetween,

a second semiconductor region SC₃ of a second conductivity type formed in a surface region of the first semiconductor region SC₁ but spaced apart from the first conductive region SC₂,

a second conductive region SC₄ formed in a surface region of the second semiconductor region SC₃ in contacting relationship forming a rectifier junction therebetween, and

a conductive gate G disposed in such a manner as to form a bridge over a barrier layer between the first semiconductor region SC₁ and the second conductive region SC₄ and between the first conductive region SC₂ and the second semiconductor region SC₃, wherein

the conductive gate G is connected to a first memory-cell-selection line,

the first conductive region SC₂ is connected to a write information selection line, and

the second conductive region SC₄ is connected to a second memory-cell-selection line.

To achieve the above object, in accordance with a sixth aspect of the invention, there is provided a semiconductor memory cell, as shown in the schematic diagram of FIG. 16(A), comprising:

a first semiconductor region SC₁ of a first conductivity type formed in a surface region of a semiconductor substrate or on an insulating substrate,

a first conductive region SC₂ formed in a surface region of the first semiconductor region SC₁ in contacting relationship forming a rectifier junction therebetween,

a second semiconductor region SC₃, of a second conductivity type opposite to the first conductivity type, formed in a surface region of the first semiconductor region SC₁ but spaced apart from the first conductive region SC₂,

a second conductive region SC₄ formed in a surface region of the second semiconductor region SC₃ in contacting relationship forming a rectifier junction therebetween, and

a conductive gate G disposed in such a manner as to form a bridge over a barrier layer between the first semiconductor region SC₁ and the second conductive region SC₄ and between the first conductive region SC₂ and the second semiconductor region SC₃, wherein

the conductive gate G is connected to a first memory-cell-selection line, and

the first semiconductor region SC₁ is connected to a second memory-cell-selection line.

In the semiconductor memory cell according to the fifth and sixth aspects of the invention, the first semiconductor region SC₁ (corresponding to the channel forming region Ch₂), the first conductive region SC₂ (corresponding to a source/drain region), the second semiconductor region SC₃ (corresponding to a source/drain region), and the conductive gate G constitute the switching transistor TR₂. Likewise, the second semiconductor region SC₃ (corresponding to the channel forming region Ch₁), the first semiconductor region SC₁ (corresponding to a source/drain region), the second conductive region SC₄ (corresponding to a source/drain region), and the conductive gate G constitute the information storage transistor TR₁.

The semiconductor channel layer or semiconductor channel forming region can be formed from a silicon, GaAs, or the like, using a known method. Each conductive gate can be formed from a metal, an impurity-added or doped silicon or polysilicon, a silicide, highly doped GaAs, or the like, using a known method. Each barrier layer can be formed from SiO₂, Si₃ N₄, Al₂ O₃, GaAlAs, or the like, using a known method. The conductive layers, conductive regions, or semiconductor regions can be formed from a doped silicon or polysilicon, a silicide, a highly doped GaAs, or the like, using a known method.

In the semiconductor memory cell of the present invention, one conductive gate of the information storage transistor and one conductive gate of the switching transistor are connected to the first memory-cell-selection line. The first memory-cell-selection line, therefore, need not be provided more than one, and the chip area can be reduced.

In the semiconductor memory cell of the first and second aspects of the present invention, the fourth conductive layer is connected to the second conductive gate. When writing information, the switching transistor conducts, and the information is stored in the form of a potential or charge on the second conductive gate of the information storage transistor. The threshold voltage of the information storage transistor required at the first conductive gate, when reading information, varies depending on the potential or charge (information) stored on the second conductive gate. This is because the space-charge regions overlap across the channel layer near the first and second gates. Therefore, when reading information, the operation of the information storage transistor can be controlled by applying an appropriately selected potential to the first conductive gate. Information reading is accomplished by sensing the operating condition (for example, magnitude of the channel current) of the information storage transistor.

In the information storage transistor, a conductive gate is provided opposite each of the two principal surfaces of the semiconductor channel layer. This structure stabilizes the operation of the information storage transistor and facilitates short-channel transistor design. This also eliminates the need for a large capacitor as required in prior art DRAMs.

The gate threshold voltage of the switching transistor is controlled by the potential applied to the read/write selection line. When writing information, the switching transistor is turned on in order to store the potential or charge on the second conductive gate, and when the write operation is completed, it is turned off. The stored information is retained as a potential or charge on the second conductive gate until the information is read out.

In the semiconductor memory cell according to the second aspect of the present invention, the switching transistor has the fourth conductive gate. This serves to further stabilize the operation of the switching transistor. This is achieved by applying to the read/write selection line a signal for putting the switching transistor in an off condition without fail during information read operation.

In the preferred modes of the semiconductor memory cell according to the first and second aspects of the present invention, the fifth conductive layer is provided which forms a rectifier junction with the first conductive layer. The provision of this fifth conductive layer serves to prevent without fail a current from flowing into the information storage transistor during the writing of information.

In the semiconductor memory cell according to the third and fourth aspects of the present invention, the fourth conductive region of the switching transistor is connected to the channel forming region of the information storage transistor. In the semiconductor memory cell according to the fifth aspect of the invention, the second semiconductor region, which forms a source/drain region of the switching transistor, corresponds to the channel forming region of the information storage transistor TR₁.

When writing information, the switching transistor conducts, and the information is stored as a potential or charge in the channel forming region of the information storage transistor. The threshold voltage of the information storage transistor required at the first conductive gate (in the third and fourth aspects of the invention) or the conductive gate (in the fifth aspect of the invention) for read operations varies depending on the potential or charge (information) stored in the channel forming region. Therefore, when reading information, by applying an appropriately selected potential to the first conductive gate or the conductive gate, the stored information state of the information storage transistor can be identified by the magnitude of the channel current (including zero current). Information reading is accomplished by sensing the operating condition of the information storage transistor.

In the semiconductor memory cell according to the sixth aspect of the present invention, the second semiconductor region SC₃, which forms a source/drain region of the switching transistor, corresponds to the channel forming region Ch₁ of the information storage transistor. Further, the first semiconductor region SC₁ that corresponds to the channel forming region of the switching transistor and also to a source/drain region of the information selection transistor is connected to the second memory-cell-selection line.

The threshold voltage of the information storage transistor required at each conductive gate for read operations can be varied by appropriately selecting the potential applied to the second memory-cell-selection line. Therefore, the on/off conditions of the information storage transistor and the switching transistor can be controlled by appropriately selecting the potential applied to the first memory-cell-selection line.

When writing information, the first line is set at a potential high enough to turn on the switching transistor, and the capacitor between the regions SC₁ and SC₃ of the switching transistor is charged depending on the potential on the second line. As a result, the information is stored in the channel forming region (the second semiconductor region SC₃) of the information storage transistor in the form of a charge or a potential difference from the region SC_(l).

When reading information, the region SC₁ is supplied with a read potential, and the potential or charge (information) stored in the channel forming region of the information storage transistor is converted to a charge or a potential difference between the second semiconductor region SC₃, which corresponds to the channel forming region Ch₁, and the second conductive region SC₄, which corresponds to a source/drain region. The threshold voltage of the information storage transistor required at the conductive gate varies depending on the charge (information). Therefore, the on/off operations of the information storage transistor can be controlled, when reading information, by applying an appropriately selected potential to the conductive gate. Information reading is accomplished by sensing the operating condition of the information storage transistor.

In the semiconductor memory cell of the invention, the stored information is retained in the form of a potential, potential difference, or charge, but since the stored information decays with time because of leakage currents due to junction leaks, etc., refreshing is necessary, and the memory cell operates in the same manner as other DRAM cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the principle of operation of a semiconductor memory cell according to a first aspect of the present invention.

FIG. 2 is a schematic cross-sectional view of a portion of a semiconductor memory cell according to Embodiment 1 of the present invention.

FIG. 3 is a diagram showing a modified example of Embodiment 1.

FIG. 4 is a diagram showing the principle of operation of a semiconductor memory cell according to a preferred mode of the first aspect of the invention.

FIG. 5 is a schematic cross-sectional view of a portion of a semiconductor memory cell according to Embodiment 2 of the present invention.

FIG. 6 is a diagram showing a modified example of Embodiment 2.

FIG. 7 is a diagram showing the principle of operation of a semiconductor memory cell according to a second aspect of the present invention.

FIG. 8 is a schematic cross-sectional view of a portion of a semiconductor memory cell according to Embodiment 3 of the present invention.

FIG. 9 is a diagram showing a modified example of Embodiment 3.

FIG. 10 is a diagram showing the principle of operation of a semiconductor memory cell according to a preferred mode of the second aspect of the present invention.

FIG. 11 is a schematic cross-sectional view of a portion of a semiconductor memory cell according to Embodiment 4 of the present invention.

FIGS. 12A to 12C shows the principle of operation of a semiconductor memory cell according to a third aspect of the present invention along with schematic cross-sectional views showing a portion thereof.

FIG. 13 is a schematic cross-sectional view of a portion of a semiconductor memory cell according to a modified example of the third aspect of the present invention.

FIGS. 14A to 14C shows the principle of operation of a semiconductor memory cell according to a fourth aspect of the present invention along with schematic cross-sectional views showing a portion thereof.

FIGS. 15A and 15B shows the principle of operation of a semiconductor memory cell according to a fifth aspect of the present invention along with a schematic cross-sectional view showing a portion thereof.

FIGS. 16A to 16C shows the principle of operation of a semiconductor memory cell according to a sixth aspect of the present invention along with schematic cross-sectional views showing a portion thereof.

FIG. 17 is a conceptual diagram showing a prior art one-transistor memory cell.

FIG. 18 is a cross-sectional view of a memory cell having a prior art trench capacitor cell structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The semiconductor memory cell of the invention will be described below in accordance with preferred embodiments thereof.

EMBODIMENT 1

Embodiment 1 is concerned with a semiconductor memory cell according to a first aspect of the invention. The semiconductor memory cell, the principle of operation of which is shown in FIG. 1 and cross sections of a portion of which are shown schematically in FIGS. 2 and 3, comprises an information storage transistor TR_(l) and a switching transistor TR₂.

The information storage transistor TR₁ comprises a semiconductor channel layer Ch₁, a first conductive gate G₁, a second conductive gate G₂, and first and second conductive layers, L₁ and L₂, each connected to either end of the semiconductor channel layer Ch₁. The switching transistor TR₂ comprises a semiconductor channel forming region Ch₂, a third conductive gate G₃, and third and fourth conductive layers, L₃ and L₄, each formed in the surface area of the semiconductor channel forming region Ch₂ in contacting relationship forming a rectifier junction therewith. The third conductive gate G₃ is formed in such a manner as to bridge the third conductive layer L₃ and the fourth conductive layer L₄.

The semiconductor channel layer Ch₁ has two opposing principal surfaces, the first principal surface MS₁ and the second principal surface MS₂. The first conductive gate G₁ is formed opposite the principal surface MS₁ of the semiconductor channel layer with a first barrier layer BL₁ interposed therebetween. Likewise, the second conductive gate G₂ is formed opposite the principal surface MS₂ of the semiconductor channel layer with a second barrier layer BL₂ interposed therebetween. The semiconductor channel forming region Ch₂ has a third principal surface MS₃. The third conductive gate G₃ is formed opposite the third principal surface MS₃ of the semiconductor channel forming region Ch₂ with a third barrier layer BL₃ interposed therebetween.

The fourth conductive layer L₄ is connected to the second conductive gate G₂. The first conductive gate G₁ and the third conductive gate G₃ are connected to a first memory-cell-selection line (for example, a word line). In the structural example shown in FIG. 2, the first conductive gate G₁ and the third conductive gate G₃ are common. The first conductive layer L₁ and the third conductive layer L₃ are connected to a second memory-cell-selection line (for example, a bit line). The second conductive layer L₂ is connected to a fixed potential including zero potential. The semiconductor channel forming region Ch₂ is connected to a read/write selection line. The read/write selection line may be a common well or a substrate. In Embodiment 1, a common well is used. The second memory-cell-selection line (for example, a bit line) need not be provided more than one, and the chip area can be reduced.

The conductive layers may be formed from a low-resistivity semiconductor, a silicide, a two-layered structure of silicide and semiconductor, a metal or the like. The barrier layers serve as barriers to channel carriers, and may be formed from an insulating material or a wide-gap semiconductor material.

In Embodiment 1, the information storage transistor TR₁ has an SOI structure. That is, with the multilayered structure shown in FIG. 2, the total area that the information storage transistor and the switching transistor take up can be made approximately equal to the area that one transistor takes up, thus allowing the chip area to be reduced.

The operation of the semiconductor memory cell will be described below, taking for example a case in which the information storage transistor TR₁ and the switching transistor TR₂ are both n-type transistors.

Potentials applied to the various lines for a memory write are designated as follows:

First memory-cell-selection line (e.g., word line) : V_(W)

Second memory-cell-selection line (e.g., bit line)

"0" write : V₀

"1" write: V_(l)

Read/write selection line: V_(B) ₋₋ W

Potentials applied to the various lines for a memory read are designated as follows. Note that during a read cycle, the read/write selection line is reverse biased.

First memory-cell-selection line (e.g., word line) : V_(R)

Read/write selection line: V_(B) ₋₋ R

For read/write, the fixed potential to which the second conductive layer L₂ is connected is designated as follows:

Fixed potential to which the second conductive layer L₂ is connected: V₂

The threshold voltages of the information storage transistor TR₁ required at the first conductive gate G₁ for memory read/write operations are designated as follows:

"0" read/write : V_(TH1) ₋₋ 0

"1" read/write : V_(TH1) ₋₋ 1

The potential of the second conductive gate G₂ is different between a "0" read/write and a "1" read/write. As a result, the threshold voltage of the information storage transistor TR₁ required at the first conductive gate G₁ differs between a "0" read/write and a "1" read/write. The information storage transistor TR₁ is provided with two conductive gates, G₁ and G₂, on opposite sides of the semiconductor channel layer Ch₁ ; this structure serves to stabilize the operation of the information storage transistor TR₁, and facilitates short-channel design. Furthermore, a capacitor having as large capacitance as required in prior art DRAMs is not required.

The thresheld voltage of the switching transistor TR₂ required at the third conductive gate G₃ for a write operation is designated as V_(TH2) ₋₋ W. Further, the threshold voltage of the switching transistor TR₂ required at the third conductive gate G₃ for a read operation is designated as V_(TH2) ₋₋ R. The threshold voltage of the switching transistor TR₂ required at the third conductive gate G₃ is different between memory write and memory read because the potential applied to the read/write selection line differs between write and read operations.

The relationships between the various potentials are set as follows: ##EQU1##

The operation of the semiconductor memory cell of Embodiment 1 will be described below.

Information write

When writing information "0" (second line potential: V₀) or "1" (second line potential: V₁), the potential of the first line is V_(W). Therefore, the potential at the third conductive gate G₃ of the switching transistor TR₂ is also V_(W). Since the potential of the read/write selection line is V_(B) ₋₋ W, the threshold voltage, V_(TH2) ₋₋ W, of the switching transistor TR₂ required at the third conductive gate has the following relationship with respect to V_(W).

    V.sub.W >V.sub.TH2.sub.-- W

As a result, the switching transistor TR₂ is ON. Therefore, the potential at the second conductive gate G₂ of the information transistor TR₁ is V₀ (when writing information "0") or V₁ (when writing information "1").

When writing information, the potential at the first conductive gate G₁ of the information storage transistor TR₁ is V_(W). The threshold voltage of the information storage transistor TR₁ required at the first conductive gate G₁ is V_(TH1) ₋₋ 0 or V_(TH1) ₋₋ 1. If the relationship

    V.sub.W >V.sub.TH1.sub.-- 0>V.sub.TH1.sub.-- 1

exists between these potentials, the information storage transistor TR₁ is also ON, but writing is performed whether the information storage transistor TR₁ is ON or OFF.

As described above, when writing information "0" or "1", the potential at the second conductive gate G₂ of the information storage transistor TR₁ is V₀ or V₁. That is, the second conductive gate G₂ is held at a potential corresponding to the information "0" or "1", and this condition is substantially maintained within a prescribed time until the information is read out. During the information retention period after the information is written and before the information is read out, the various portions of the information storage transistor TR₁ and the switching transistor TR₂ are set at such potentials that neither transistor will conduct.

Information read

When reading information "0" or "1", the potential of the first line is V_(R). Therefore, the potential at the third conductive gate G₃ of the switching transistor TR₂ is V_(R). Since the potential of the read/write selection line is V_(B) ₋₋ R, a reverse bias is applied between the source and the channel forming region of the switching transistor TR₂. V_(B) ₋₋ R is set so that the threshold voltage, V_(TH2) ₋₋ R, of the switching transistor TR₂ required at the third conductive gate has the following relationship with respect to V_(R).

    V.sub.TH2.sub.-- R>V.sub.R

The switching transistor TR₂ is thus held OFF.

When reading information, the potential at the first conductive gate G₁ of the information storage transistor TR₁ is V_(R). The threshold voltage of the information storage transistor TR_(l) at the first conductive gate G₁ is V_(TH1) ₋₋ 0 or V_(TH1) ₋₋ 1. The threshold voltage of the information storage transistor TR₁ is dependent on the potential of the second conductive gate G₂. The following relationship exists between these potentials.

    V.sub.TH1.sub.-- 0>V.sub.R >V.sub.TH1.sub.-- 1

Therefore, when the stored information is "0", the information storage transistor TR₁ is OFF. On the other hand, when the stored information is "1", the information storage transistor TR₁ is ON.

Thus, the information storage transistor TR₁ is set to an ON or OFF condition, depending on the stored information. Therefore, when the stored information is "1", current flows to the second line; when the stored information is "0", no current but the leakage current flows to the second line. The stored information can be read in this manner by the information storage transistor TR₁.

The above-described operating conditions of the information storage transistor TR₁ and the switching transistor TR₂ are summarized in Table 1. The potential values given in Table 1 are only illustrative examples, and each potential can take any value as long as it satisfies the above conditions.

                  TABLE 1                                                          ______________________________________                                         Units: volts                                                                   Memory write   "0" write        "1" write                                      ______________________________________                                         1st line potential                                                                            V.sub.W 2.0      V.sub.W                                                                              2.0                                      2nd line potential                                                                            V.sub.0 0        V.sub.1                                                                              1.0                                      Read/write line        V.sub.B-W      V.sub.B-W                                Threshold of TR.sub.2                                                                         V.sub.TH2-W                                                                            0.5      V.sub.TH2-W                                                                          0.5                                      required at 3rd gate                                                           3rd gate potential                                                                            V.sub.W 2.0      V.sub.W                                                                              2.0                                      Condition of TR.sub.2                                                                         ON               ON                                             2nd gate potential                                                                            V.sub.0 0        V.sub.1                                                                              1.0                                      Threshold of TR.sub.1                                                                         V.sub.TH1-0                                                                            1.1      V.sub.TH1-1                                                                          0.5                                      required at 1st gate                                                           1st gate potential                                                                            V.sub.W 2.0      V.sub.W                                                                              2.0                                      condition of TR.sub.1                                                                         ON               ON                                             ______________________________________                                         Memory read    "0" read         "1" read                                       ______________________________________                                         1st line potential                                                                            V.sub.R 1.0      V.sub.R                                                                              1.0                                      Read/write line        V.sub.B-R      V.sub.B-R                                Threshold of TR.sub.2                                                                         V.sub.TH2-R                                                                            1.5      V.sub.TH2-R                                                                          1.5                                      required at 3rd gate                                                           3rd gate potential                                                                            V.sub.R 1.0      V.sub.R                                                                              1.0                                      Condition of TR.sub.2                                                                         OFF              OFF                                            2nd gate potential                                                                            V.sub.0 0        V.sub.1                                                                              1.0                                      Threshold of TR.sub.1                                                                         V.sub.TH1-0                                                                            1.1      V.sub.TH1-1                                                                          0.5                                      required at 1st gate                                                           1st gate potential     1.0            1.0                                      Condition of TR.sub.2                                                                         OFF              ON                                             Fixed potential                                                                               V.sub.2 0        V.sub.2                                                                              0                                        2nd line current                                                                              OFF              ON                                             ______________________________________                                    

A modified example of the semiconductor memory cell of Embodiment 1 illustrated in FIG. 2 is shown schematically in cross section in FIG. 3. In the semiconductor memory cell shown in FIG. 2, the first conductive gate G₁ and the third conductive gate G₃ are common. By contrast, in the semiconductor memory cell shown in FIG. 3, the second conductive gate G₂ and the fourth conductive layer L₄ are common. Also, the read/write selection line is formed from a well. While a slightly larger plan area is required as compared with the memory cell shown in FIG. 2, the memory cell shown in FIG. 3 have the advantages of relatively smooth surface topography, which is advantageous in lithography, and a less number of contacts.

EMBODIMENT 2

Embodiment 2 is concerned with a preferred mode of the semiconductor memory according to the first aspect of the invention. The semiconductor memory cell, the principle of operation of which is shown in FIG. 4 and cross sections of a portion of which are shown schematically in FIGS. 5 and 6, comprises an information storage transistor TR₁ and a switching transistor TR₂. The structures of the information storage transistor TR₁ and the switching transistor TR₂ are fundamentally the same as those of Embodiment 1, except for the points hereinafter described.

The information transistor TR₁ is formed from a transistor of a first conductivity type, for example, a p-type transistor, and the switching transistor TR₂ is formed from a transistor of the opposite conductivity type to that of the information storage transistor TR₁, for example, an n-type transistor. In this case, if the first conductive layer L₁ and the second conductive layer L₂ are formed from semiconductor material, then their conductivity type should be p type. Likewise, if the first conductive gate G₁, the second conductive gate G₂, the third conductive gate G₃, the third conductive layer L₃, and the fourth conductive layer L₄ are formed from semiconductor material, then their conductivity type should be n⁺ type. Alternatively, these regions may be formed from a silicide, a two-layer structure of silicide and semiconductor, or a metal. The semiconductor channel forming region Ch₂ is connected to a second fixed potential including zero potential.

Further, the first conductive layer L₁ is connected to the second line via a fifth conductive layer L₅ which forms a rectifier junction with the first conductive layer. The provision of the fifth conductive layer serves to prevent without fail a current from flowing into the information storage transistor TR₁ during the writing of information. Furthermore, there is no possibility of the information write voltage being applied to the information storage transistor TR₁ and thereby interfering with the write operation as was the case with the prior art.

The operation of the semiconductor memory cell of Embodiment 2 will be described below.

The potentials of the information storage transistor TR₁ and the switching transistor TR₂ are set to satisfy the following relationships.

    V.sub.W >V.sub.TH2.sub.-- W

    V.sub.TH1.sub.-- 1>V.sub.R >V.sub.TH1.sub.-- 0

Information write

When writing information "0" (second line potential: V₀) or "1" (second line potential: V₁), the potential of the first line is V_(W) (>0). Therefore, the potential at the third conductive gate G₃ of the switching transistor TR₂ is also V_(W) (>0). Since the second fixed potential is V_(B) ₋₋ W, if V_(W) is set as

    V.sub.W >V.sub.TH2.sub.-- W+V.sub.0 or V.sub.1

where V_(TH2) ₋₋ W is the threshold voltage of the switching transistor TR₂ required at the third conductive gate G₃, then the switching transistor TR₂ is ON. Therefore, the potential at the second conductive gate G₂ of the information transistor TR₁ is V₀ (when writing information "0") or V₁ (when writing information "1").

When writing information, the potential at the first conductive gate G₁ of the information storage transistor TR₁ is V_(W) (>0). Therefore, when V₀ or V₁ <V_(W) -V_(TH1) ₋₋ 1, the information storage transistor TR₁ is OFF. Even if it is ON, current flow is blocked by the presence of the rectifier junction between the first conductive layer L₁ and the fifth conductive layer L₅.

As described above, when writing information "0" or "1", the potential at the second conductive gate G₂ of the information storage transistor TR₁ is V₀ or V₁. That is, the second conductive gate G₂ is held at a potential corresponding to the information "0" or "1", and this condition is substantially maintained until the information is read out. During the information retention period after the information is written and before the information is read out, the various portions of the information storage transistor TR₁ and the switching transistor TR₂ are set at such potentials that neither transistor will conduct.

Information read

When reading information "0" or "1", the potential of the first line is V_(R) (<0). Therefore, the potential at the third conductive gate G₃ of the switching transistor TR₂ is V_(R) (<0), and the switching transistor TR₂ remains OFF unless the potential of the second line is made more negative than V_(R) -V_(TH2) ₋₋ R. Normally, for a read operation, the second line is set at a small potential and information is read by sensing the current.

The potential at the first conductive gate G₁ of the information storage transistor TR₁ is V_(R) (<0). The threshold voltage of the information storage transistor TR₁ required at the first conductive gate G₁ is V_(TH1) ₋₋ 0 or V_(TH1) ₋₋ 1. The threshold voltage of the information storage transistor TR₁ is dependent on the potential of the second conductive gate G₂. The following relationship exists between these potentials.

    V.sub.TH1.sub.-- 1>V.sub.R >V.sub.TH1.sub.-- 0

Therefore, when the stored information is "0", the information storage transistor TR₁ is ON. On the other hand, when the stored information is "1", the information storage transistor TR₁ is OFF.

Thus, the information storage transistor TR₁ is set to an ON or OFF condition, depending on the stored information. The second conductive layer L₂ of the information storage transistor TR₁ is connected to the fixed potential (V₂); therefore, when the stored information is "0", current flows to the second line, and when the stored information is "1", current does not flow to the second line. The stored information can be read in this manner by the information storage transistor TR₁.

A cross section of a portion of the semiconductor memory cell of Embodiment 2 is shown schematically in FIG. 5. The semiconductor memory cell of Embodiment 2 is fundamentally the same as the semiconductor memory cell shown in FIG. 2, except that the information storage transistor TR₁ and the switching transistor TR₂ are of different conductivity types and that the first conductive layer L₁ is connected to the second line via the n-type fifth conductive layer L₅ which forms a rectifier junction with the p-type first conductive layer L_(l).

Table 2 summarizes the operating conditions of the semiconductor memory cell in which the information storage transistor TR₁ is formed from a p-type transistor and the switching transistor TR₂ is formed from an n-type transistor.

                  TABLE 2                                                          ______________________________________                                         Units: volts                                                                   Memory write   "0" write        "1" write                                      ______________________________________                                         1st line potential                                                                            V.sub.W  1.5     V.sub.W                                                                               1.5                                     2nd line potential                                                                            V.sub.0  0       V.sub.1                                                                               1.0                                     Read/write line        V.sub.B-W      V.sub.B-W                                Threshold of TR.sub.2                                                                         V.sub.TH2-W                                                                             0.5     V.sub.TH2-W                                                                           0.5                                     required at 3rd gate                                                           3rd gate potential                                                                            V.sub.W  1.5     V.sub.W                                                                               1.5                                     Condition of TR.sub.2                                                                         ON               ON                                             2nd gate potential                                                                            V.sub.0  0       V.sub.1                                                                               1.0                                     1st gate potential                                                                            V.sub.W  1.5     V.sub.W                                                                               1.5                                     Condition of TR.sub.1                                                                         OFF              OFF                                            ______________________________________                                         Memory read    "0" read         "1" read                                       ______________________________________                                         1st line potential                                                                            V.sub.R -1.0     V.sub.R                                                                              -1.0                                     3rd gate potential                                                                            V.sub.R -1.0     V.sub.R                                                                              -1.0                                     Condition of TR.sub.2                                                                         OFF              OFF                                            2nd gate potential                                                                            V.sub.0  0       V.sub.1                                                                               1.0                                     Threshold of TR.sub.1                                                                         V.sub.TH1-0                                                                            -0.5     V.sub.TH1-1                                                                          -1.1                                     required at 1st gate                                                           1st gate potential     -1.0           -1.0                                     Condition of TR.sub.1                                                                         ON               OFF                                            Fixed potential    V.sub.2          V.sub.2                                    2nd line current                                                                              ON               OFF                                            ______________________________________                                    

The potential values given in Table 2 are only illustrative examples, and each potential can take any value as long as it satisfies the above conditions.

A modified example of the semiconductor memory cell illustrated in FIG. 5 is shown schematically in cross section in FIG. 6. The semiconductor memory cell shown is fundamentally the same as the semiconductor memory cell shown in FIG. 3, except that the information storage transistor TR₁ and the switching transistor TR₂ are of different conductivity types and that the first conductive layer L₁ is connected to the second line via the n-type fifth conductive layer L₅ which forms a rectifier junction with the p-type first conductive layer L₁.

EMBODIMENT 3

Embodiment 3 is concerned with a semiconductor memory cell according to a second aspect of the invention. The semiconductor memory cell, the principle of operation of which is shown in FIG. 7 and cross sections of a portion of which are shown schematically in FIGS. 8 and 9, comprises an information storage transistor TR₁ and a switching transistor TR₂. The information storage transistor TR₁ comprises a first semiconductor channel layer Ch₁, a first conductive gate G₁, a second conductive gate G₂, and first and second conductive layers, L₁ and L₂, each connected to either end of the first semiconductor channel layer Ch₁. The switching transistor TR₂ comprises a second semiconductor channel layer Ch₂, a third conductive gate G₃, a fourth conductive gate G₄, and third and fourth conductive layers, L₃ and L₄, each connected to either end of the second semiconductor channel layer Ch₂. In Embodiment 3, the information storage transistor TR₁ and the switching transistor TR₂ both have an SOI structure (including a structure in which the barrier layer is formed from a wide-gap semiconductor, for example, GaAlAs as compared with GaAs).

The first semiconductor channel layer Ch₁ has two opposing principal surfaces, the first principal surface MS₁ and the second principal surface MS₂. The first conductive gate G₁ is formed opposite the principal surface MS₁ of the first semiconductor channel layer with a first barrier layer BL₁ interposed therebetween. Likewise, the second conductive gate G₂ is formed opposite the principal surface MS₂ of the first semiconductor channel layer with a second barrier layer BL₂ interposed therebetween.

The second semiconductor channel layer Ch₂ has two opposing principal surfaces, the third principal surface MS₃ and the fourth principal surface MS₄. The third conductive gate G₃ is formed opposite the third principal surface MS₃ of the second semiconductor channel layer Ch₂ with a third barrier layer BL₃ interposed therebetween. Likewise, the fourth conductive gate G₄ is formed opposite the fourth principal surface MS₄ of the second semiconductor channel layer Ch₂ with a third barrier layer BL₄ interposed therebetween.

The fourth conductive layer L₄ is connected to the second conductive gate G₂. In Embodiment 3, the fourth conductive layer L₄ and the second conductive gate G₂ are common. The first conductive gate G₁ and the third conductive gate G₃ are connected to a first memory-cell-selection line (for example, a word line). The first conductive layer L₁ and the third conductive layer L₃ are connected to a second memory-cell-selection line (for example, a bit line). The second conductive layer L₂ is connected to a fixed potential including zero potential. The fourth conductive gate G₄ is connected to a read/write selection line.

The semiconductor memory cell of Embodiment 3 is different from the semiconductor memory cell of Embodiment 1 in that the switching transistor TR₂ has the fourth conductive gate G₄. This serves to further stabilize the operation of the switching transistor TR₂ and yet allows further miniaturization.

When the information storage transistor TR₁ and the switching transistor TR₂ are both n-type transistors, the operation of the semiconductor memory cell is the same as that described in Embodiment 1 (see Table 1), and therefore, detailed explanation thereof is not repeated here.

A modified example of the semiconductor memory cell of Embodiment 3 illustrated in FIG. 8 is shown schematically in cross section in FIG. 9. In the semiconductor memory cell shown in FIG. 8, the second conductive gate G₂ and the fourth conductive layer L₄ are common. By contrast, in the semiconductor memory cell shown in FIG. 9, the first conductive gate G₁ and the third conductive gate G₃ are formed in the same substrate surface. The first conductive gate G₁ and the third conductive gate G₃ may be connected to each other. While a slightly larger plan area is required than that of the memory cell shown in FIG. 8, the memory cell shown in FIG. 9 has the advantage that the first semiconductor channel layer Ch₁ and the second semiconductor channel layer Ch₂ can be formed using a single high-quality semiconductor layer whereas two such layers are needed in the structure of the semiconductor memory cell shown in FIG. 8. There is a further advantage that the first conductive gate G₁ and the third conductive gate G₃ can be made common.

EMBODIMENT 4

Embodiment 4 is concerned with a preferred mode of the semiconductor memory cell according to the second aspect of the invention. The semiconductor memory cell, the principle of operation of which is shown in FIG. 10 and a cross section of a portion of which is shown schematically in FIG. 11, comprises an information storage transistor TR₁ and a switching transistor TR₂. The structures of the information storage transistor TR₁ and the switching transistor TR₂ are fundamentally the same as those of Embodiment 3, except for the points hereinafter described.

The information transistor TR₁ is formed from a transistor of a first conductivity type, for example, a p-type transistor, and the switching transistor TR₂ is formed from a transistor of the opposite conductivity type to that of the information storage transistor TR₁, for example, an n-type transistor. In this case, if the first conductive layer L₁ and the second conductive layer L₂ are formed from semiconductor material, then their conductivity type should be p type. Likewise, if the first conductive gate G₁, the second conductive gate G₂, the third conductive gate G₃, the fourth conductive gate G₄, the third conductive layer L₃, and the fourth conductive layer L₄ are formed from semiconductor material, then their conductivity type should be n⁺ type. The fourth conductive gate G₄ is connected to a second fixed potential including zero potential.

Further, the first conductive layer L₁ is connected to the second line via an n-type fifth conductive layer L₅ which forms a rectifier junction with the p-type first conductive layer L₁.

A cross section of a portion of the semiconductor memory cell of Embodiment 4 is shown schematically in FIG. 11. The semiconductor memory cell of Embodiment 3 is fundamentally the same as the semiconductor memory cell shown in FIG. 9, except that the first and second conductive layers, L₁ and L₂, are of different conductivity type and that the first conductive layer L₁ is connected to the second line via the n⁺ fifth conductive layer L₅ which forms a rectifier junction with the p-type first conductive layer L₁. In FIG. 11, the fifth conductive layer L₅ and the rectifier junction between the first conductive layer L₁ and the fifth conductive layer L₅ are shown in schematic form for simplicity. It will also be appreciated that the first conductive gate G₁ and the third conductive gate G₃ may be connected to each other.

When the information storage transistor TR₁ is formed from a p-type transistor and the switching transistor TR₂ from an n-type transistor, the operation of the semiconductor memory cell is the same as that described in Embodiment 2 (see Table 2), and therefore, detailed explanation thereof is not repeated here.

EMBODIMENT 5

Embodiment 5 is concerned with a semiconductor memory cell according to a third aspect of the invention. The semiconductor memory cell, the principle of operation of which is shown in FIG. 12(A) and a cross section of a portion of which is shown schematically in FIG. 12(B), comprises an information storage transistor TR₁ formed from a field-effect transistor of a first conductivity type (for example, n type) and a switching transistor TR₂ formed from a field-effect transistor of a second conductivity type (for example, p type).

The information storage transistor TR₁ comprises a first channel forming region Ch₁ of the second conductivity type, a first conductive gate G₁ formed above the first semiconductor channel forming region Ch₁ with a first barrier layer interposed therebetween, and first and second conductive regions, SC₁ and SC₂, separated by the first conductive gate G₁. The first conductive gate G₁ is formed in such a manner as to bridge the first conductive region SC₁ and the second conductive region SC₂.

The switching transistor TR₂ comprises a second semiconductor channel forming region Ch₂ of the first conductivity type, a second conductive gate G₂ formed above the second semiconductor channel forming region Ch₂ with a second barrier layer interposed therebetween, a third conductive region SC₃, and a fourth conductive region SC₄. The second conductive gate G₂ is formed in such a manner as to bridge the third conductive region SC₃ and the fourth conductive region SC₄.

More specifically, the conductive regions are the source and drain regions. The first and second conductive regions, SC₁ and SC₂, are each formed from a low-resistivity semiconductor of the opposite conductivity type to that of the first semiconductor channel forming region Ch₁, or from a metal or a silicide that forms a rectifier junction with the first semiconductor channel forming region Ch₁. Likewise, the third and fourth conductive regions, SC₃ and SC₄, are each formed from a low-resistivity semiconductor of the opposite conductivity type to that of the second semiconductor channel forming region Ch₂, or from a metal or a silicide that forms a rectifier junction with the second semiconductor channel forming region Ch₂.

The first conductive gate G₁ of the storage transistor TR₁ and the second conductive gate G₂ of the switching transistor TR₂ are connected to a first memory-cell-selection line (for example, a word line). The fourth conductive region SC₄ of the switching transistor TR₂ is connected to, or formed common with, the channel forming region Ch₁ of the storage transistor TR₁. The third conductive region SC₃ of the switching transistor TR₂ is connected to a second memory-cell-selection line (for example, a bit line). The first conductive region SC₁ of the information storage transistor TR₁ is connected to a read line. The second conductive region SC₂ is connected to a fixed potential including zero potential.

Potentials applied to the various lines for a memory write are designated as follows:

First memory-cell-selection line (e.g., word line) : V_(W)

Second memory-cell-selection line (e.g., bit line)

"0" write: V₀

"1" write : V₁

Read potential is designated as follows:

First memory-cell-selection line (e.g., word line) : V_(R)

Also, for a memory read, the potential of the read line to which the first conductive region SC₁ of the information storage transistor TR₁ is connected is designated as follows:

Read line potential : V₂

The threshold voltages of the information storage transistor TR₁ required at the first conductive gate G₁ for read operations are designated as follows:

"0" read : V_(TH1) ₋₋ 0

"1" read : V_(TH1) ₋₋ 1 The potential of the channel forming region Ch₁ is different between a "0" read and a "1" read. As a result, the threshold voltage of the information storage transistor TR₁ required at the first conductive gate G₁ differs between a "0" read and a "1" read. However, the structure does not require the provision of a large capacitor as required in the prior art DRAM shown in FIG. 17.

The potential of the information storage transistor TR₁ is set to satisfy the following relationship.

    V.sub.TH1.sub.-- 1>V.sub.R >V.sub.TH1.sub.-- 0

The operation of the semiconductor memory cell of Embodiment 5 will be described below.

Information write

When writing information "0" (second line potential: V₀) or "1" (second line potential: V₁), the potential of the first line is set at V_(W) (<0). As a result, the potential at the second conductive gate G₂ of the switching transistor TR₂ is also V_(W) (<0). Therefore, the switching transistor TR₂ is ON. Consequently, the potential of the channel forming region Ch₁ of the information storage transistor TR₁ is V₀ (when writing information "0") or V₁ (when writing information "1". Or V_(W) -V_(TH2) when V_(W) <V₁ +V_(TH2).

When writing information, the potential at the first conductive gate G₁ of the information storage transistor TR₁ is V_(W) (<0). Therefore, the information storage transistor TR₁ is OFF. Thus, when writing information "0" or "1", the potential of the channel forming region Ch₁ of the information storage transistor TR₁ is V₀ (when writing information "0") or V₁ or V_(W) -V_(TH2) (when writing information "1"). This condition changes with time because of leakage currents (between the substrate and the semiconductor channel forming region Ch₁ of the information storage transistor TR₁, off currents of the switching transistor TR₂, etc.), but is maintained within an allowable range until the information is read out. More specifically, during the information retention period after the information is written and before the information is read out, the various portions of the information storage transistor TR₁ and the switching transistor TR₂ are set at such potentials that neither transistor will conduct. Furthermore, the so-call refresh operation is performed before the change in the potential of the channel forming region Ch₁ of the information storage transistor TR₁ becomes large enough to cause errors in read operations.

Information read

When reading information "0" or "1", the potential of the first line is V_(R) (>0). As a result, the potential at the second conductive gate G₂ of the switching transistor TR₂ is V_(R) (>0), and the switching transistor TR₂ is OFF.

The potential at the first conductive gate G₁ of the information storage transistor TR₁ is V_(R) (>0). Also, the threshold voltage of the information storage transistor TR₁ required at the first conductive gate G₁ is V_(TH1) ₋₋ 0 or V_(TH1) ₋₋ 1. The threshold voltage of the information storage transistor TR₁ is dependent on the potential of the channel forming region Ch₁. The following relationship exists between these threshold voltages.

    V.sub.TH1.sub.-- 1>V.sub.R >V.sub.TH1.sub.-- 0

Therefore, when the stored information is "0", the information storage transistor TR₁ is ON. On the other hand, when the stored information is "1", the information storage transistor TR₁ is OFF.

Thus, the information storage transistor TR₁ is set to an ON or OFF condition, depending on the stored information. Since the first conductive region SC₁ of the information storage transistor TR₁ is connected to the read line or the second line, current flows or does not flow to the information storage transistor TR₁, depending on the stored information ("0" or "1"). The stored information can be read out in this manner by the information storage transistor TR₁. A mode in which the first conductive region SC₁ of the information storage transistor TR₁ is connected to the second line will be described hereinafter.

The above-described operating conditions of the information storage transistor TR₁ and the switching transistor TR₂ are summarized in Table 3. The potential values given in Table 3 are only illustrative examples, and each potential can take any value as long as it satisfies the above conditions.

                  TABLE 3                                                          ______________________________________                                         Units: volts                                                                   Memory write "0" write         "1" write                                       ______________________________________                                         1st line potential                                                                          V.sub.W -3.0      V.sub.W                                                                              -3.0                                      2nd line potential                                                                          V.sub.0 0         V.sub.1                                                                              -2.0                                      2nd gate potential                                                                          V.sub.W -3.0      V.sub.W                                                                              -3.0                                      Condition of TR.sub.2                                                                       ON                ON                                              Potential of channel                                                                        V.sub.0 0         V.sub.1                                                                              -2.0                                      forming region                                                                 1st gate potential                                                                          V.sub.W -3.0      V.sub.W                                                                              -3.0                                      Condition of TR.sub.1                                                                       OFF               OFF                                             ______________________________________                                         Memory read  "0" read          "1" read                                        ______________________________________                                         1st line potential                                                                          V.sub.R 1.0       V.sub.R                                                                              1.0                                       2nd gate potential                                                                          V.sub.R 1.0       V.sub.R                                                                              1.0                                       Condition of TR.sub.2                                                                       OFF               OFF                                             Potential of channel                                                                        V.sub.0 0         V.sub.1                                                                              -2.0                                      forming region                                                                 Threshold of TR.sub.1                                                                       V.sub.TH1-0                                                                            0.5       V.sub.TH1-1                                                                          1.1                                       required at 1st gate                                                           1st gate potential   1.0             1.0                                       Condition of TR.sub.1                                                                       ON                OFF                                             Read line potential                                                                         0.5 V*            0.5 V*                                          ______________________________________                                          *2nd line potential: 1 V for FIG. 12(c)                                  

In Embodiment 5, it is possible to omit the read line. More specifically, the read line may be formed common with the second line. In this case, the first conductive region SC₁ is connected to the second line via a p-n junction or a Schottky junction type rectifying diode D₂, as indicated by a dotted line in FIG. 12(B). This semiconductor memory cell structure can be realized, for example, in the structure shown in FIG. 12(C). In the structure shown in FIG. 12(C), the diode D₂ is formed by forming the first conductive region SC₁ from a semiconductor and forming the third conductive region SC₃ in the surface area thereof, the conductivity type of the third conductive region SC₃ being opposite to that of the first conductive region SC₁. Furthermore, in the semiconductor memory cell shown in FIG. 12(C), the first semiconductor channel forming region Ch₁ and the fourth conductive region SC₄ are formed of a common region. Likewise, the first conductive region SC₁ and the second semiconductor channel forming region Ch₂ are formed of a common region.

In the thus structured semiconductor memory cell, the readout voltage applied to the second line must be small enough that a large forward current will not flow across the junction between the first conductive region SC₁ and the third-conductive region SC₃ (0.4 V or less in the case of a p-n junction); otherwise, latch-up may occur. One way to prevent the latch-up condition is to form the third conductive region SC₃ from a silicide or a metal so that the third conductive region SC₃ and the first conductive region SC₁ form a junction such as a Schottky junction where majority carriers essentially form the forward current.

As a structure that can utilize the conventional MOS technology, the third conductive region SC₃ may be formed, as shown in FIG. 13, for example, from a p-type semiconductor region SC_(3P) and a metal layer of Mo, Al, etc. or a silicide layer, SC_(3S), that can form a Schottky junction with the first conductive region SC₁. In the semiconductor memory cell shown in FIG. 13, the first conductive gate G_(l) and the second conductive gate G₂ are formed common. Also, the reference sign E designates an electrode Al or Al/TiN/Ti triple layer formed on the third conductive region SC₃, and the first conductive region SC₁ is formed, for example, from an n-type semiconductor.

Thus, this embodiment provides the structure that can also be employed in CMOS fabrication, and therefore, the structure, among others, the structure shown in FIG. 12(B), is particularly suitable for DRAMs integrated in CMOS ASICs.

EMBODIMENT 6

Embodiment 6 is concerned with a semiconductor memory cell according to a fourth aspect of the invention. The semiconductor memory cell of Embodiment 6, the principle of operation of which is shown in FIG. 14(A) and a cross section of a portion of which is shown schematically in FIG. 13(B), comprises an information storage transistor TR₁ of a first conductivity type (for example, n type) and a switching transistor TR₂ of a second conductivity type (for example, p type) opposite to the first conductivity type.

The information storage transistor TR₁ comprises a first conductive gate G₁, first and second conductive regions, SC₁ and SC₂, formed, for example, from an n⁺ semiconductor, and a first semiconductor channel forming region Ch₁ (common with a p-well in the example shown in FIG. 14(B)). The first conductive region SC₁ and the second conductive region SC₂ each form a rectifier junction with the surface area of the first semiconductor channel forming region Ch₁. The first conductive gate G₁ is formed opposite a first principal surface of the first semiconductor channel forming region Ch₁, with a first barrier layer interposed therebetween, in such a manner as to bridge the first and second conductive regions SC₁ and SC₂.

The switching transistor TR₂ comprises a second conductive gate G₂, third and fourth conductive regions, SC₃ and SC₄, formed, for example, from a p⁺ semiconductor, and a second semiconductor channel forming region Ch₂. The third conductive region SC₃ and the fourth conductive region SC₄ are connected to the second semiconductor channel forming region Ch₂. The second conductive gate G₂ is formed opposite a second principal surface of the second semiconductor channel forming region Ch₂ with a second barrier layer interposed therebetween.

The first conductive gate G₁ of the information storage transistor TR₁ and the second conductive gate G₂ of the switching transistor TR₂ are connected to a first memory-cell-selection line (for example, a word line). The fourth conductive region SC₄ of the switching transistor TR₂ is connected to the channel forming region Ch₁ of the information storage transistor TR₁. The third conductive region SC₃ of the switching transistor TR₂ is connected to a second memory-cell-selection line (for example, a bit line). The second conductive region SC₂ of the information storage transistor TR₁ is connected to a fixed potential. The first conductive region SC₁ of the information storage transistor TR₁ is connected to the third conductive region SC₃ of the switching transistor TR₂, forming at their boundary a rectifier junction of p-n junction or Schottky junction type. With the impurity concentration in the first conductive region SC₁ at the junction adjusted to 2×10¹⁹ cm⁻³ or less, the dielectric strength or leakage current characteristic at the rectifier junction can be improved.

Potentials applied to the various lines for a memory write are designated as follows:

First memory-cell-selection line (e.g., word line) : V_(W)

Second memory-cell-selection line (e.g., bit line)

"0" write: V₀

"1" write: V_(l)

Read potential is designated as follows:

First memory-cell-selection line (e.g., word line) : V_(R)

Also, for a memory read, the fixed potential to which the second conductive region SC₂ of the information storage transistor TR₁ is connected is designated as follows:

Fixed potential : V₂

The threshold voltages of the information storage transistor TR₁ required at the first conductive gate G₁ for read operations are designated as follows:

"0" read : V_(TH1) ₋₋ 0

"1" read : V_(TH1) ₋₋ 1

The potential of the channel forming region Ch₁ is different between a "0" read and a "1" read. As a result, the threshold voltage of the information storage transistor TR₁ required at the first conductive gate G₁ differs between a "0" read and a "1" read. However, the structure does not require the provision of a large capacitor as required in the prior art DRAM.

The potential of the information storage transistor TR₁ is set to satisfy the following relationship.

    V.sub.TH1.sub.-- 1>V.sub.R >V.sub.TH1.sub.-- 0

The operation of the semiconductor memory cell of Embodiment 6 will be described below.

Information write

When writing information "0" (second line potential: V₀) or "1" (second line potential: V₁), the potential of the first line is set at V_(W) (<0). As a result, the potential at the second conductive gate G₂ of the switching transistor TR₂ is also V_(W) (<0). Therefore, the switching transistor TR₂ is ON. Consequently, the potential of the channel forming region Ch₁ of the information storage transistor TR₁ is V₀ (when writing information "0") or V₁ (when writing information "1". Or V_(W) -V_(TH2) when V_(W) <V₁ +V_(TH2).

When writing information, the potential at the first conductive gate G₁ of the information storage transistor TR₁ is V_(W) (<0). Therefore, the information storage transistor TR₁ is OFF. Thus, when writing information "0" or "1", the potential of the channel forming region Ch₁ of the information storage transistor TR₁ is V₀ (when writing information "0") or V₁ or V_(W) -V_(TH2) (when writing information "1"). This condition changes with time because of leakage currents (between the substrate and the semiconductor channel forming region Ch₁ of the information storage transistor TR₁, off currents of the switching transistor TR₂, etc.), but is maintained within an allowable range until the information is read out. More specifically, during the information retention period after the information is written and before the information is read out, the various portions of the information storage transistor TR₁ and the switching transistor TR₂ are set at such potentials that neither transistor will conduct. Furthermore, the so-call refresh operation is performed before the change in the potential of the channel forming region Ch₁ of the information storage transistor TR₁ becomes large enough to cause errors in read operations.

The first conductive region SC₁ of the information storage transistor TR_(l) is connected to the third conductive region SC₃ of the switching transistor TR₂, forming at their boundary a rectifier junction of p-n junction or Schottky junction type. This perfectly prevents the current flow to the first conductive region SC₁ during the writing of information.

Information read

When reading information "0" or "1", the potential of the first line is V_(R) (>0). As a result, the potential at the second conductive gate G₂ of the switching transistor TR₂ is V_(R) (>0), and the switching transistor TR₂ is OFF.

The potential at the first conductive gate G₁ of the information storage transistor TR₁ is V_(R) (>0). Also, the threshold voltage of the information storage transistor TR₁ required at the first conductive gate G_(l) is V_(TH1) ₋₋ 0 or V_(TH1) ₋₋ 1. The threshold voltage of the information storage transistor TR₁ is dependent on the potential of the channel forming region Ch₁. The following relationship exists between these threshold voltages.

    V.sub.TH1.sub.-- 1>V.sub.R >V.sub.TH1.sub.-- 0

Therefore, when the stored information is "0", the information storage transistor TR₁ is ON. On the other hand, when the stored information is "1", the information storage transistor TR₁ is OFF.

Thus, the information storage transistor TR₁ is set to an ON or OFF condition, depending on the stored information. Since the second conductive region SC₂ of the information storage transistor TR₁ is connected to the fixed potential, current flows or does not flow to the information storage transistor TR₁, depending on the stored information ("0" or "1"). The stored information can be read out in this manner by the information storage transistor TR₁. The above-described operating conditions of the information storage transistor TR₁ and the switching transistor TR₂ are summarized in Table 4. The potential values given in Table 4 are only illustrative examples, and each potential can take any value as long as it satisfies the above conditions.

                  TABLE 4                                                          ______________________________________                                         Units: volts                                                                   Memory write "0" write         "1" write                                       ______________________________________                                         1st line potential                                                                          V.sub.W -2.5      V.sub.W                                                                              -2.5                                      2nd line potential                                                                          V.sub.0 0         V.sub.1                                                                              -2.0                                      2nd gate potential                                                                          V.sub.W -2.5      V.sub.W                                                                              -2.5                                      Condition of TR.sub.2                                                                       ON                ON                                              Potential of TR.sub.1                                                                       V.sub.0 0         V.sub.1                                                                              -1.0                                      channel forming                                                                region                                                                         1st gate potential                                                                          V.sub.W -2.5      V.sub.W                                                                              -2.5                                      Condition of TR.sub.1                                                                       OFF               OFF                                             ______________________________________                                         Memory read  "0" read          "1" read                                        ______________________________________                                         1st line potential                                                                          V.sub.R 0.9       V.sub.R                                                                              0.9                                       2nd gate potential                                                                          V.sub.R 0.9       V.sub.R                                                                              0.9                                       Condition of TR.sub.2                                                                       OFF               OFF                                             Potential of TR.sub.1                                                                       V.sub.0 0         V.sub.1                                                                              -2.0                                      channel forming                                                                region                                                                         Threshold of TR.sub.1                                                                       V.sub.TH1-0                                                                            0.5       V.sub.TH1-1                                                                          1.1                                       required at 1st gate                                                           1st gate potential   0.9             0.9                                       Condition of TR.sub.1                                                                       ON                OFF                                             2nd line potential                                                                          V.sub.2 (≃1.0)                                                                     V.sub.2 (≃1.0)                    ______________________________________                                    

FIG. 14(C) shows a modified example of the semiconductor memory cell of Embodiment 6. In the semiconductor memory cell shown in FIG. 14(C), an auxiliary gate (a third conductive gate) G₃ is formed opposite the second conductive gate G₂ and facing a third principal surface of the second semiconductor channel forming region Ch₂ with a third barrier layer interposed therebetween. The auxiliary gate G₃ has the function of shielding the semiconductor forming region Ch₂ from potentials induced at and ions adhering to the surface of the semiconductor memory cell, thereby ensuring stable operation of the semiconductor memory cell. Furthermore, by connecting the auxiliary gate G₃ to the read/write selection line, read or write operating margins can be improved. The semiconductor memory cell thus constructed has further improved characteristics as compared with the semiconductor memory cell whose principle of operation is shown in FIG. 4.

EMBODIMENT 7

Embodiment 7 is concerned with a semiconductor memory cell according to a fifth aspect of the invention. The semiconductor memory cell of Embodiment 7, the principle of operation of which is shown in FIG. 15(A) and a cross section of a portion of which is shown schematically in FIG. 15(B), comprises: a first semiconductor region SC₁ of a first conductivity type (for example, n type); a first conductive region SC₂ formed from a material of a second conductivity type opposite to the first conductivity type (for example, p⁺) or from other material such as a metal or a silicide that forms a rectifier junction with the first semiconductor region SC₁ ; a second semiconductor region SC₃ of the second conductivity type (for example, p⁺ type); a second conductive region SC₄ formed from a material of the first conductivity type (for example, n⁺) or from other material such as a metal or a silicide that forms a rectifier junction with the second semiconductor region SC₃ ; and a conductive gate G disposed in such a manner as to form a bridge over a barrier layer between the first semiconductor region SC₁ and the second conductive region SC₄ and between the first conductive region SC₂ and the second semiconductor region SC₃.

The first semiconductor region SC₁ is formed in the surface area of a semiconductor substrate or on an insulating substrate. The first conductive region SC₂ is formed in the surface area of the first semiconductor region SC₁. The second semiconductor region SC₃ is formed in the surface area of the first semiconductor region SC₁ but spaced apart from the first conductive region SC₂. The second conductive region SC₄ is formed in the surface area of the second semiconductor region SC₃.

The conductive gate G is connected to a first memory-cell-selection line (for example, a word line). The first conductive region SC₂ is connected to a write information setting line. The second conductive region SC₄ is connected to a second memory-cell-selection line (for example, a bit line).

The first semiconductor region SC₁ (corresponding to the channel forming region Ch₂), the first conductive region SC₂ (corresponding to a source/drain region), the second semiconductor region SC₃ (corresponding to a source/drain region), and the conductive gate G constitute the switching transistor TR₂. Likewise, the second semiconductor region SC₃ (the surface area below the conductive gate G corresponds to the channel forming region Ch₁ (p)), the first semiconductor region SC₁ (corresponding to a source/drain region), the second conductive region SC₄ (corresponding to a source/drain region), and the conductive gate G constitute the information storage transistor TR₁.

Potentials applied to the various lines for a memory write are designated as follows:

First memory-cell-selection line (e.g., word line) : V_(W)

Write information setting line

"0" write : V₀

"1" write : V₁

Read potential is designated as follows:

First memory-cell-selection line (e.g., word line) : V_(R)

Also, for a memory read, the potential of the second memory-cell-selection line (for example, bit line) to which the second conductive region SC₄ is connected is designated as follows:

Second memory-cell-selection line: V₂ A fixed potential, including zero potential, is applied to the first semiconductor region SC_(l).

The threshold voltages of the information storage transistor TR₁ required at the conductive gate G for read operations are designated as follows:

"0" read : V_(TH1) ₋₋ 0

"1" read : V_(TH1) ₋₋ 1

The potential of the channel forming region Ch₁ is different between a "0" read and a "1" read. As a result, the threshold voltage of the information storage transistor TR₁ at the conductive gate G differs between a "0" read and a "1" read. However, the structure does not require the provision of a large capacitor as required in the prior art DRAM.

The potential of the information storage transistor TR₁ is set to satisfy the following relationship.

    V.sub.TH1.sub.-- 1>V.sub.R >V.sub.TH1.sub.-- 0

The operation of the semiconductor memory cell of Embodiment 7 will be described below.

Information write

When writing information "0" (write information setting line potential: V₀) or "1" (write information setting line potential: V₁), the potential of the first line is set at V_(W) (<0). As a result, the potential at the conductive gate G of the switching transistor TR₂ is also V_(W) (<0). Therefore, the switching transistor TR₂ is ON. Consequently, the potential of the channel forming region Ch₁ of the information storage transistor TR₁ is V₀ (when writing information "0") or V₁ (when writing information "1". Or V_(W) -V_(TH2) when V_(W) <V₁ +V_(TH2).

During the information retention period after the information is written and before the information is read out, the various portions of the information storage transistor TR₁ and the switching transistor TR₂ are set at such potentials that neither transistor will conduct. To achieve this, the potential of the first line should be set at 0 (V) and the potential of the second line at V₁, for example.

When writing information, the potential at the conductive gate G of the information storage transistor TR₁ is V_(W) (<0). Therefore, the information storage transistor TR₁ is OFF. Thus, when writing information "0" or "1", the potential of the channel forming region Ch₁ of the information storage transistor TR₁ is V₀ (when writing information "0") or V_(l) or V_(W) -V_(TH2) (when writing information "1"). This condition changes with time because of leakage currents (between the first semiconductor region SC₁ and the semiconductor channel forming region Ch₁ of the information storage transistor TR₁, off currents of the switching transistor TR₂, etc.), but is maintained within an allowable range until the information is read out. The so-call refresh operation is performed before the change in the potential of the channel forming region Ch₁ of the information storage transistor TR₁ becomes large enough to cause errors in read operations.

Information read

When reading information "0" or "1", the potential of the first line is V_(R) (>0). As a result, the potential at the conductive gate G of the switching transistor TR₂ is V_(R) (>0), and the switching transistor TR₂ is OFF.

The potential at the conductive gate G of the information storage transistor TR₁ is V_(R) (>0). Also, the threshold voltage of the information storage transistor TR₁ required at the conductive gate G is V_(TH1) ₋₋ 0 or V_(TH1) ₋₋ 1. The threshold voltage of the information storage transistor TR₁ is dependent on the potential of the channel forming region Ch₁. The following relationship exists between these potentials.

    V.sub.TH1.sub.-- 1>V.sub.R >V.sub.TH1.sub.-- 0

Therefore, when the stored information is "0", the information storage transistor TR₁ is ON. On the other hand, when the stored information is "1", the information storage transistor TR₁ is OFF.

Thus, the information storage transistor TR₁ is set to an ON or OFF condition, depending on the stored information. Since the second conductive region SC₄ is connected to the second line, current flows or does not flow to the information storage transistor TR₁, depending on the stored information ("0" or "1"). The stored information can be read out in this manner by the information storage transistor TR₁.

The above-described operating conditions of the information storage transistor TR₁ and the switching transistor TR₂ are summarized in Table 5. The potential values given in Table 5 are only illustrative examples, and each potential can take any value as long as it satisfies the above conditions.

                  TABLE 5                                                          ______________________________________                                         Units: volts                                                                   Memory write "0" write         "1" write                                       ______________________________________                                         1st line potential                                                                          V.sub.W -3.0      V.sub.W                                                                              -3.0                                      Potential of write                                                                          V.sub.0 0         V.sub.1                                                                              -2.0                                      information setting                                                            line                                                                           Gate potential                                                                              V.sub.W -3.0      v.sub.W                                                                              -3.0                                      Condition of TR.sub.2                                                                       ON                ON                                              Potential of channel                                                                        V.sub.0 0         V.sub.1                                                                              -2.0                                      forming region                                                                 Condition of TR.sub.1                                                                       OFF               OFF                                             ______________________________________                                         Memory read  "0" read          "1" read                                        ______________________________________                                         1st line potential                                                                          V.sub.R 1.0       V.sub.R                                                                              1.0                                       Gate potential                                                                              V.sub.R 1.0       V.sub.R                                                                              -1.0                                      Condition of TR.sub.2                                                                       OFF               OFF                                             Potential of V.sub.0 0         V.sub.1                                                                              -2.0                                      channel forming                                                                region                                                                         Threshold of TR.sub.1                                                                       V.sub.TH1-0                                                                            0.5       V.sub.TH1-1                                                                          1.1                                       required at 1st gate                                                           Condition of TR.sub.1                                                                       ON                OFF                                             2nd line potential                                                                          0.5               0.5                                             ______________________________________                                    

EMBODIMENT 8

Embodiment 8 is concerned with a semiconductor memory cell according to a sixth aspect of the invention. The semiconductor memory cell of Embodiment 8, the principle of operation of which is shown in FIG. 16(A) and a cross section of a portion of which is shown schematically in FIG. 16(B), comprises: a first semiconductor region SC₁ of a first conductivity type (for example, n type); a first conductive region SC₂ formed from a material of a second conductivity type opposite to the first conductivity type (for example, p⁺) or from other material such as a metal or a silicide that forms a rectifier junction with the first semiconductor region SC₁); a second semiconductor region SC₃ of the second conductivity type (for example, p⁺ type); a second conductive region SC₄ formed from a material of the first conductivity type (for example, n⁺) or from other material such as a metal or a silicide that forms a rectifier junction with the second semiconductor region SC₃); and a conductive gate G formed in such a manner as to bridge the second conductive region SC₄ and a portion of the first semiconductor region SC₁ flanked by the second semiconductor region SC₃ and the first conductive region SC₂ and also bridge the first conductive region SC₂ and the second semiconductor region SC₃. The semiconductor memory cell shown in FIG. 16(B) is depicted in more detail in FIG. 16(C). For further embodiment of the semiconductor memory cell structure shown in FIG. 15(B), part of the structure of the semiconductor memory cell structure shown in FIG. 16(C) can be applied to the structure shown in FIG. 15(B).

The first semiconductor region SC₁ is formed on a semiconductor substrate as an isolated region in the surface area thereof, or on an insulating substrate as a region attached to a support substrate with an insulating layer such as an SiO₂ layer interposed therebetween. The first conductive region SC₂ is formed in the surface area of the first semiconductor region SC₁. The second semiconductor region SC₃ is formed in the surface area of the first semiconductor region SC₁ but spaced apart from the first conductive region SC₂. The second conductive region SC₄ is formed in the surface area of the second semiconductor region SC₃.

The conductive gate G is connected to a first-memory-cell selection line (for example, a word line). The first semiconductor region SC₁ is connected to a second memory-cell-selection line (for example, a bit line). The first conductive region SC₂ is connected to a first fixed potential. The second conductive region SC₄ is connected to a second fixed potential.

The first semiconductor region SC₁ (corresponding to the channel forming region Ch₂), the first conductive region SC₂ (corresponding to a source/drain region), the second semiconductor region SC₃ (corresponding to a source/drain region), and the conductive gate G constitute the switching transistor TR₂. Likewise, the second semiconductor region SC₃ (the surface area below the conductive gate G corresponds to the channel forming region Ch₁ (p)), the first semiconductor region SC₁ (corresponding to a source/drain region), the second conductive region SC₄ (corresponding to a source/drain region), and the conductive gate G constitute the information storage transistor TR₁.

The first fixed potential to which the first conductive region SC₂ is connected is designated as V₃ (≦0). Likewise, the second fixed potential to which the second conductive region SC₄ is connected is designated as V₄ (≧0).

Potentials applied to the various lines for a memory write are designated as follows:

First memory-cell-selection line (e.g., word line) : V_(W)

Second memory-cell-selection line (e.g., bit line)

"0" write : V₀

"1" write : V₁

Likewise, the threshold voltages of the switching transistor TR₂ required at the conductive gate for write operations are designated as follows:

"0" write: V_(TH2W) ₋₋ 0

"1" write: V_(TH2W) ₋₋ 1

Read potential is designated as follows:

First memory-cell-selection line (e.g., word line) : V_(R)

Also, the threshold voltages of the information storage transistor TR₁ required at the conductive gate G for read operations are designated as follows:

"0" read : V_(TH1R) ₋₋ 0

"1" read : V_(TH1R) ₋₋ 1

Generally, the potentials of the channel forming regions Ch₁ and Ch₂ are different between a "0" read/write and a "1" read/write. As a result, the threshold voltages of the information storage transistor TR₁ and the switching transistor TR₂ required at the conductive gate G differ between a "0" read/write and a "1" read/write. However, the structure does not require the provision of a large capacitor as required in the prior art DRAM.

The potential at the conductive gate of the switching transistor TR₂, relative to V_(W), is set as shown below, for example, for simplicity of explanation.

    V.sub.W >V.sub.TH2W.sub.-- 1 or V.sub.TH2W.sub.-- 0 whichever larger

The potential at the conductive gate of the information storage transistor TR₁, relative to V_(R), is set as follows:

    V.sub.TH1.sub.-- 1>V.sub.R >V.sub.Th1.sub.-- 0

The operation of the semiconductor memory cell of Embodiment 8 will be described below.

Information write

When writing information "0" (second line potential: V₀) or "1" (second line potential: V₁), the potential of the first line is set at V_(W) (<0). As a result, the potential at the conductive gate G of the switching transistor TR₂ is also V_(W) (<0). V_(W) is

    V.sub.W >V.sub.TH2W.sub.-- 1 or V.sub.TH2W.sub.-- 0 whichever larger

Therefore, for a write operation, the switching transistor TR₂ is ON. Consequently, the potential of the channel forming region CH₁ of the information storage transistor TR₁ is

"0" information write: V₃

"1" information write: V₃

When writing information, the potential at the conductive gate G of the information storage transistor TR₁ is V_(W) (<0). Therefore, the information storage transistor TR₁ is OFF. Thus, when writing information "0" or "1", the potential of the channel forming region Ch₁ of the information storage transistor TR₁ is V₃ for both "0" and "1" information write. During the retention period, since TR₂ is also OFF, when the second line potential at this time is denoted by V₅ V₅ ≦V₀ or V₁) the potential of the first semiconductor channel Ch₁ is γ{V₃ -(V₀ -V₅)} or γ{V₃ -(V₁ -V₅)}, where γ is the ratio of the capacitance between the first semiconductor region SC₁ and the second semiconductor region SC₃ to the total capacitance between the second semiconductor region SC₃ and the other areas (including the first semiconductor region SC₁) than the second semiconductor region SC₃. This condition changes with time because of leakage currents (between the first semiconductor region SC₁ and the semiconductor channel forming region Ch₁ of the information storage transistor TR₁, off currents of the switching transistor TR₂, etc.), but is maintained within an allowable range until the information is read out.

During the information retention period after the information is written and before the information is read out, the various portions of the information storage transistor TR₁ and the switching transistor TR₂ are set at such potentials that neither transistor will conduct. To achieve this the potential of the first line should be set at 0 (V) and the potential of the second line at V₅, for example. The so-called refresh operation is performed before the change in the potential of the channel forming region Ch₁ of the information storage transistor TR₁ becomes large enough to cause errors in read operations.

Information read

When reading information "0" or "1", the potential of the first line is V_(R) (>0), and the potential of the second line is V₆ (V₆ ≦V₀ or V₁). As a result, the potential at the conductive gate G of the switching transistor TR₂ is V_(R) (>0), and the switching transistor TR₂ is OFF.

The potential at the conductive gate G of the information storage transistor TR₁ is V_(R) (>0). Also, the threshold voltage of the information storage transistor TR₁ required at the first conductive gate G₁ is V_(TH1R) ₋₋ 0 or V_(TH1R) ₋₋ 1 which is given by the potential between the second semiconductor region SC₃ and the second conductive region SC₄ when γ{V₃ -(V₀ -V₆)} or γ{V₃ -(V₁ -V₆)} is met. The threshold voltage of the information storage transistor TR₁ is dependent on the potential of the channel forming region Ch₁. The following relationship exists between these potentials.

    V.sub.TH1.sub.-- 0>V.sub.R >V.sub.TH1.sub.-- 1

Therefore, when the stored information is "0", the information storage transistor TR₁ is OFF. On the other hand, when the stored information is "1", the information storage transistor TR₁ is ON.

Thus, the information storage transistor TR₁ is set to an ON or OFF condition, depending on the stored information. Since the second conductive region SC₄ is connected to the second fixed potential, current flows or does not flow to the information storage transistor TR₁, depending on the stored information ("0" or "1"). The stored information can be read out in this manner by the information storage transistor TR₁.

The above-described operating conditions of the information storage transistor TR₁ and the switching transistor TR₂ are summarized in Table 6. The potential values given in Table 6 are only illustrative examples, and each potential can take any value as long as it satisfies the above conditions.

                  TABLE 6                                                          ______________________________________                                         Units: volts                                                                   Memory write "0" write     "1" write                                           ______________________________________                                         1st line potential                                                                          V.sub.W  -2.0     V.sub.W                                                                               -2.0                                     2nd line potential                                                                          V.sub.0  2.0      V.sub.1                                                                               0                                        1st fixed potential   V.sub.2         V.sub.2                                  Gate potential                                                                              V.sub.W  -2.0     V.sub.W                                                                               -2.0                                     Thereshold of TR.sub.2                                                                      V.sub.TH2W-0                                                                            -1.2     V.sub.TH2W-1                                                                          -0.5                                     required at gate                                                               Condition of TR.sub.2                                                                       ON            ON                                                  Potential of TR.sub.1 1st fixed       1st fixed                                channel forming       potential       potential                                region                e.g. 0          e.g. 0                                   Condition of TR.sub.1                                                                       OFF           OFF                                                 ______________________________________                                         Memory read  "0" read      "1" read                                            ______________________________________                                         1st line potential                                                                          V.sub.R  1.0      V.sub.R                                                                               1.0                                      Gate potential                                                                              V.sub.R  1.0      V.sub.R                                                                               1.0                                      Condition of TR.sub.2                                                                       OFF           OFF                                                 Potential of TR.sub.1 -0.75           +0.25                                    channel forming                                                                region                                                                         Threshold of TR.sub.1                                                                       V.sub.TH1R-0                                                                            0.85     V.sub.TH1R-1                                                                          0.5                                      required at 1st gate                                                           Condition of TR.sub.1                                                                       OFF           ON                                                  2nd line potential                                                                          0.5           0.5                                                 2nd fixed potential                                                                         0.25          0.25                                                ______________________________________                                    

The semiconductor memory cell of the invention has been described above in accordance with preferred embodiments, but it will be appreciated that the invention is not limited to those described preferred embodiments. For example, in the first and second aspects of the invention described in Embodiments 1 and 3, the information storage transistor TR₁ and the switching transistor TR_(l) may be both formed from p-type transistors. Furthermore, in the preferred modes of the first and second aspects of the invention described in Embodiments 2 and 4, the information storage transistor TR₁ may be formed from an n-type transistor and the switching transistor TR₂ from a p-type transistor. Moreover, in the third to fifth aspects described in Embodiments 5 to 8, the information storage transistor TR₁ may be formed from a p-type transistor and the switching transistor TR₂ from an n-type transistor. It will also be appreciated that the arrangement of elements in each transistor shown is only illustrative and may be modified as needed.

The present invention is applicable not only to memory cells formed from silicon semiconductors but also to memory cells formed from compound semiconductors such as GaAs, for example.

In the semiconductor memory cell of the present invention, the operation of the information storage transistor is determined in depending relationship on the potential or charge (information) stored on the second conductive gate or in the channel forming region of the information storage transistor, and the information read out as the transistor current during refresh intervals is independent of the magnitude of the capacitance of a capacitor (for example, the capacitance of the second conductive gate plus added capacitance, etc.) if such a capacitor is added. This therefore serves to solve the problem of capacitor capacitance encountered in prior art semiconductor memory cells; if a capacitor is added for refresh interval adjustment, a very large capacitor such as those required in the prior art DRAMs is not required. Furthermore, the maximum area of the semiconductor memory cell is equal to or smaller than the area of two transistors.

In the semiconductor memory cell according to the first and second aspects of the invention, a conductive gate is provided opposite each of two principal surfaces of the semiconductor channel layer of the information storage transistor. This structure eliminates the problem of unstable transistor operation inherent in prior art transistor structures; that is, the structure of the invention is effective in stabilizing the operation of the information storage transistor, and facilitates short-channel transistor design. The first conductive gate and the third conductive gate are both connected to the first memory-cell-selection line. Accordingly, the first memory-cell-selection line need not be provided more than one, and the chip area can be reduced. Furthermore, depending on the structure, it is possible to form a memory cell within an area equal to one transistor area.

In the semiconductor memory cell according to the second aspect of the invention, a fourth gate is provided as an extra gate of the switching transistor. The provision of this extra gate serves to further stabilize the operation of the switching transistor.

In the preferred modes of the semiconductor memory cell according to the first and second aspects of the invention, a fifth conductive layer is provided. The provision of this fifth conductive layer prevents without fail a current from flowing to the information storage transistor during the information write operation.

The semiconductor memory cell fabrication process according to the third aspect of the invention is compatible with the fabrication process for CMOS ASICs, as shown, for example, in FIG. 12(B). The process therefore allows DRAM functions to be incorporated in an ASIC without requiring extra processing steps although the semiconductor memory cell area increases slightly. On the other hand, in the semiconductor memory cell structure shown in FIG. 12(C), for example, one extra processing step may be required as compared to the CMOS ASIC structure, but the semiconductor memory cell area can be reduced nearly by half.

According to the fourth aspect of the invention, the semiconductor memory cell can be implemented on an area approximately equal to one transistor area.

According to the fifth and sixth aspects of the invention, the semiconductor memory cell can be implemented on an area approximately equal to one transistor area by using conventional semiconductor memory cell fabrication techniques without relying on SOI technology. 

What is claimed is:
 1. A semiconductor memory cell comprising:an information storage transistor comprising a semiconductor channel layer having first and second opposing principal surfaces; first and second conductive gates respectively disposed opposite said two principal surfaces of said semiconductor channel layer with first and second barrier layers respectively interposed therebetween; and first and second conductive layers each one connected to an opposite end of said semiconductor channel layer, and a switching transistor comprising a semiconductor channel forming region having a third principal surface; a third conductive gate disposed opposite said third principal surface of said semiconductor channel forming region with a third barrier layer interposed therebetween; and third and fourth conductive layers each formed in a surface region in contacting relationship therewith and forming a rectifier junction, said third conductive gate being formed to partially overlap said third conductive layer and said fourth conductive layer, wherein said fourth conductive layer is connected to said second conductive gate, said first conductive gate and said third conductive gate are connected to a first memory-cell-selection line, said first conductive layer and said third conductive layer are connected to a second memory-cell-selection line, said second conductive layer is connected to a fixed potential including zero potential, and said semiconductor channel forming region is connected to a read/write selection line.
 2. A semiconductor memory cell according to claim 1, whereinsaid information storage transistor is formed from a transistor of a first conductivity type, said switching transistor is formed from a transistor of a conductivity type opposite to the first conductivity type, said first conductive layer is connected to said second line via a fifth conductive layer which forms a rectifier junction with said first conductive layer, and said semiconductor channel forming region is connected to a second fixed potential including zero potential. 